This invention relates generally to a processor for a data processing system, and, in particular, to an instruction set modifier register which is loadable under program control and which permits the execution of more than one instruction set by the processor, utilizing the same instruction decoding circuitry and instruction execution control logic circuitry.
In a data processing system the basic logical and arithmetic computational operations are performed by the processor. For this purpose there is provided within the processor a number of registers and logic circuits. The processor registers are used to receive, hold, and transmit information (data and instructions) used by the processor. Several different types of registers are provided within the typical processor. For example, an accumulator register temporarily stores data and accumulates the results of logical or arithmetic operations. A program counter stores the address of the next instruction in memory to be executed. An instruction register stores the instruction code (also known as the operation code) portion of the instruction which is currently being executed by the processor, and an address register or data counter stores the operand portion of the currently executing instruction. Throughout the specification the term "instruction" will hereafter be used to refer specifically to the instruction code or operation code portion of an instruction.
To enable the processor to perform all of the intended arithmetic and logic operations which are desired to be performed, the processor is provided with the capability of executing a repertory of individual instructions collectively known as an instruction set. Individual instructions are executed by the processor to perform such operations as loading information into a register, transferring information between registers or between registers and memory, comparing the contents of two registers, and so forth. Such instructions may be thought of as "macroinstructions" since the execution of one such instruction by the processor comprises a number of sub-operations or "microinstructions" by the circuitry making up the instruction execution control logic portion of the processor. During the execution of a single instruction many different logic gates in the instruction execution control logic circuitry may be opened and closed in a precise sequence in order to implement the particular macro-operation called for by the instruction. The opening or closing of each gate may be individually viewed as a single microinstruction.
It is very useful for a processor to have the capability of executing an instruction set for a different computer in addition to its own instruction set. Normally the processor is endowed with a unique instruction set comprising a plurality of individual operation code words, each of which comprises a distinctive combination of 1's and 0's, in response to which the instruction decoding circuitry and instruction execution control circuitry perform all of the individual microinstructions necessary to carry out the particular instruction. A known prior art emulation technique, referred to as microprogramming, allows a processor to execute instruction sets from several different computers. For example, the IBM System/360 and System/370, as well as other known computer systems, utilize a special program called a "microprogram" to execute a plurality of individual microinstructions which together comprise a basic macroinstruction. In response to a given macroinstruction, a special processor memory known as a control store is accessed and a microprogram corresponding to the macroinstruction is executed, with each of the individual microinstructions serving to control the desired operation of the instruction execution control logic circuitry. In this manner, an IBM 1401, for example, can have its instructions emulated on an IBM System/360 or System/370.
It will be appreciated that the capability of a processor to emulate another processor by executing the instruction set of the other processor adds a great deal of flexibility and versatility to a computer system, which can result ultimately in substantial savings to the user of such system.
The advantages to the user of a data processing system having emulation capabilities include the ability to execute software written for more than one type of computer system thereby avoiding the necessity of expensive and time-consuming software conversion efforts.
While the technique of microprogramming has been utilized in a number of mainframe and minicomputer systems, it does not lend itself well to microcomputer systems, where the space necessary for a control store is difficult to justify on the limited silicon area of a large-scale-integrated (LSI) circuit device. There is therefore a need for a mechanism in a processor permitting the processor to execute two or more instruction sets without requiring the extensive consumption of silicon space in an LSI circuit device.